The Firefox team has more or less said that the feature is not coming to desktop. I think a primitive version was hidden behind a flag, but even that is going to be dropped.
I use it for about half a dozen apps. Azure Portal, Youtube Music, SoundCloud, Google Cloud Platform, Twitter, Teamwork, and this chat app I maintain. And I keep them pinned to my taskbar. I love this feature.
> As Gijs says, we have limited resources and so have to spend those resources on work that appears to have the most impact on our mission. Based on the available data we have (both the research we performed as well as looking at how Chrome and Edge's implementations are being received) PWAs on desktop fall behind other work right now.
https://bugzilla.mozilla.org/show_bug.cgi?id=1682593
Firefox for Android has this. Are you referring to Firefox for desktop?